DirectEd

This is like... amazing... You can edit graphics in Gameboy, NES, Virtual Boy, Neo Geo Pocket Color, SNES/PC Engine, Sega Master System/Gamegear/Wonder Swan Color, and Sega Genesis format. Yeah, that's a big list. It has a friendly TLayer-esque format and is just all around good stuff to play with. You'll need Windows and a recent version of DirectX on your computer to use it. Try it and I guarantee you'll like it.